It looks better, and the gameplay doesn't change at all if the graphics are better looking. I run everything on ultra and it looks great and plays the same. Actually there is only 1 unit that can cause my frame-rate to drop a noticeable amount. Ground-Mode Vikings, if there are a bunch, 15+ on screen firing their Gatling guns my frame rate dips, it's very strange as it doesn't happen in any other situation.
If your FPS drops at all then it does affect gameplay. Even if the only situation where your FPS drops happens 1 in every 20 games, that's not worth it and you're better off dropping your settings.
It may not happen right away, but eventually that FPS drop is going to cost you a game. If you're just playing casually and don't actually care about trying to play optimally then I guess it doesn't matter.
But for anyone that finds the game more enjoyable while winning and would rather not give away an unnecessary game, setting your graphics to a level where you will get no (playable reduction, measurable level of) FPS drop is going to be optimal.
[edit]
I guess I should disclose that I am a textbook type-A and don't find joy in doing anything unless I feel I'm doing it completely optimally. Even while playing SC2 "casually" in the sense of time investment, I have to make sure the time I do spend is best alloted because I'd rather be better at it than not.
Most competitive people I know share the same feelings.